Leukocyte antigen CD37 (“CD37”), also known as GP52-40, tetraspanin-26, or TSPAN26, is a transmembrane protein of the tetraspanin superfamily (Maecker et al., 1997 FASEB J. 11:428-442). It is a heavily glycosylated protein with four transmembrane domains that is expressed on B cells during the pre-B to peripheral mature B-cell stages, but is absent on terminal differentiation to plasma cells. (Link et al., 1987, J Pathol. 152:12-21). The CD37 antigen is only weakly expressed on T-cells, myeloid cells and granulocytes (Schwartz-Albiez et al. 1988, J. Immunol., 140(3)905-914). However. CD37 is also expressed on malignant B-cells such as those founding non-Hodgkin's lymphoma (NHL) and chronic lymphoid leukemia (CLL) (Moore et al. 1986, J Immunol. 137(9):3013-8). This expression profile suggests that CD37 represents a promising therapeutic target for B-cell malignancies.
While the exact physiological role of CD37 in unclear, studies suggest a potential role in T-cell proliferation (van Spriel et al. 2004, J Immunol., 172(5):2953-61) As part of the tetraspanin family of cell surface glycoproteins, CD37 may also complex with other surface proteins (Angelisová 1994. Immunogenetics., 39(4):249-56). Mice deficient in CD37 expression were developed and revealed no changes in development and cellular composition of lymphoid organs. Only reduced levels of IgG1 and alterations of responses to T-cell dependent antigens were observed (Knobeloch et al. 2000, Mol Cell Biol., 20(15):5363-9).
Antibodies are emerging as a promising method to treat such cancers. In particular, antibodies that are able to induce apoptosis in target cells are desirable. In addition, antibodies having complement-dependent cytotoxicity (CDC) activity and antibody-dependent cytotoxicity (ADCC) are also desirable.
Currently, an anti-CD20 antibody called rituximab is being used to treat B-cell malignancies (Leget et al., 1998, Curr. Opin. Oncol., 10:548-551). However, only a subset of patients respond to rituximab treatment, and even responding patients taking rituximab eventually relapse and often develop resistance to rituximab treatment. In addition. CD37-binding agents are also being tested as potential therapeutics for B-cell malignancies. Trubion Pharmaceuticals developed the CD37-binding agents SMIP-016 and TRU-016 (Zhao et al., 2007, Blood, 110:2569-2577). SMIP-016 is a single chain polypeptide that includes variable regions from a hybridoma and engineered human constant regions. TRU-016 is a humanized version of the anti-CD37 SMIP protein. See e.g. U.S. Published Application No. 2007/0009519. TRU-016 is being tested clinically for the treatment of chronic lyphocytic leukemia (CLL). Boehringer Ingelheim has also disclosed a CD37 binding agent in International Published Application No. WO 2009/019312. However, no CDC activity has been described for any of these binding agents and no in vitro pro-apoptotic activity has been described in the absence of cross-linking agents.
Radio-immunotherapy (RIT) has been attempted using a radio-labeled anti-CD37 antibody MB-1 in two separate trials. Therapeutic doses of 131I-MB-1 were administered to six relapsed NHL patients (Press et al. 1989 J Clin Oncol. 7(8): 1027-38, Press at el. 1993, N Engl J Med. 329(17): 1219-24). All six patients achieved a complete remission (CR) with a duration of four to thirty-one months. In another trial, 131I-MB-1 was administered to ten relapsed NHL patients (Kaminski et al. 1992 J Clin Oncol. 10(11):1696-711). A total of four patients had a response ranging in duration from two to six months, although only one CR was reported. However, not all patients could be treated due to an unfavorable biodistribution of the radio-label which raised concern for radiation exposure of vital non-target organs. Indeed. RIT related toxicities were observed in these trials including severe myelosupression and cardiopulmonary toxicity. While these clinical data suggest that anti-CD37 radio-immunoconjugates may be effective, these therapies are cumbersome to administer, and at relapse post-RIT patients cannot be retreated with RIT due to the risks associated with high doses of radiation.
To overcome the limitations of RIT, antibody-cytotoxic agent conjugates (ACC), also called antibody-drug conjugates (ADC), have been developed. These are immunoconjugates that include a cytotoxic agent covalently linked to an antibody through a chemical linker which can allow for specific delivery of cytotoxic drugs to cells expressing a protein recognized by the antibody. However, proteins that are poorly internalized are not considered to be favorable targets for such therapeutics. CD37 is structurally similar to CD20 as both antigens contain four transmembrane domains, although CD20 is not part of the tetraspanin family (Tedder et al. 1989, J. Immun. 142: 2560-2568). Antibodies against several B-cell antigens including CD37 and CD20 have been studied for their ability to undergo endocytosis and degradation (Press et al. 1989. Cancer Res. 49(17):4906-12, and Press et al. 1994. Blood. 83(5):1390-7). The anti-CD37 antibody MB-1 was retained on the cell surface and internalized slowly in Daudi lymphoma cells in vitro. The MB-1 antibody also had a low rate of endocytosis and intracellular metabolism in NHL patient cells in vitro. Similar results were obtained with the anti-CD20 antibody IFS, which was also retained mainly on the lymphoma cell surface and internalized poorly. ADCs of CD20 antibodies have been studied previously but have not demonstrated significantly strong potency, especially when non-disulfide or acid stable linkers are used (see for example Polson et al., 2009, Cancer Res., 69(6):2358-2364). In light of these observations, CD37 has not been considered a favorable target for antibody-drug conjugates.
Therefore, there exists a need for CD37 binding agents including antibodies, antigen-binding fragments thereof, and antibody-drug conjugates (immunoconjugates) as a means to treat B-cell malignancies. The present invention addresses that need.